In recent years increasing numbers of oil and gas fields have been developed in offshore areas. The oil and gas produced from such fields must be transported to shore either by pipeline or tanker. In utilizing tankers for this purpose, it is typical to produce the oil and gas through a riser extending from the seafloor to a surface loading facility from which produced hydrocarbons can be transferred to a waiting tanker. To avoid having to terminate hydrocarbon production when a transport tanker is not present to receive flow, it is common practice to locate a hydrocarbon storage unit at the surface loading facility. Most commonly, this storage unit is an unpowered, permanently moored storage tanker.
The use of storage tankers presents difficulties in regions where severe weather or ice floes occur. The forces exerted on the storage tanker mooring system by storm conditions or an ice floe can be quite severe, often many orders of magnitude greater than the forces present under ordinary conditions. Providing a mooring system capable of withstanding such extreme conditions poses a formidable technical challenge. Accordingly, most mooring systems adapted for arctic use provide some mechanism for releasing the vessel from the mooring system once environmental forces reach a predetermined level. Upon release, the vessel is allowed to drift until the adverse conditions abate, at which time it is returned to the mooring site and re-moored.
One of the earliest mooring systems based on this concept utilizes mooring lines extending from both the bow and stern of the storage tanker to anchors located at the ocean floor. The mooring lines are oriented such that the vessel is maintained on a fixed heading into the prevailing wind and waves and is situated above the riser. When environmental conditions become sufficiently severe, the mooring lines are buoyed off and the storage tanker moved. A disadvantage of this system, especially in the Arctic, is that the vessel cannot rotate to head into ice, wind and waves approaching abeam of its fixed heading. This forces the vessel to move off station in conditions which a ship able to alter its heading could weather.
To avoid the problems resulting from maintaining a vessel on a set heading, while retaining the ability to keep the vessel at a fixed location, turret mooring systems were developed. A typical turret mooring system is described in U.S. Pat. No. 3,605,668, issued Sept. 20, 1971. In this system the vessel is provided with a turret which is fixedly positioned relative to the ocean floor by a number of releasable mooring lines. The vessel weathervanes about the turret to assume the heading of least resistance to existing environmental conditions. Because the mooring lines enter the turret from a submerged location beneath the vessel, access to the points of mooring line attachment is awkward. To release the vessel it is necessary either to buoy-off and release each mooring line or to pull each mooring line into the vessel. This causes significant delays in releasing and re-mooring the vessel.
An alternative mooring system, known as the single point mooring system, utilizes a single surface buoy moored to the ocean floor. The storage tanker is moored to the buoy rather than directly to the ocean floor. A production riser extends from the ocean floor to a flowline swivel on the buoy. A loading hose extends between the swivel and the vessel. As the direction of the wind and waves changes, the vessel can weathervane about the buoy to maintain the heading of least resistance. The buoy to vessel attachment is above the ocean surface, simplifying release and reattachment. A disadvantage of the single point mooring system is that it is necessary to provide some means of preventing the tanker from overriding the surface buoy in high seas. The most widely practiced solution to this problem involves the use of a rigid mooring arm or yoke to maintain the vessel a fixed distance from the buoy. Further, the buoy, which remains at a fixed position at the surface even when the storage vessel has moved off, must be able to withstand any ice floes or other environmental conditions acting upon it. A typical single point mooring system is described in U.S. Pat. No. 4,371,037, issued Feb. 1, 1983.
In yet another type of mooring system, detailed in U.S. Pat. No. 4,321,720, issued Mar. 30, 1982, a buoyant mooring station is anchored to remain submerged a preselected distance beneath the ocean surface. To onload produced hydrocarbons, a tanker positions itself above the mooring station and lowers a flowline. The flowline is coupled to the mooring station for transferring hydrocarbons to the tanker. The tanker remains on station through use of dynamic positioning. While this system substantially eliminates the action of storms, waves and ice floes on the mooring station, it is disadvantageous in that the tanker can take on produced hydrocarbons only in relatively calm conditions. Because this system can support only moderate forces acting on the tanker, it is not well suited for applications in which it is desirable to interrupt oil production as infrequently as possible.
It would be advantageous to provide a mooring system for use in the Arctic and other areas with adverse environmental conditions which could maintain a storage tanker on location in all but the most extreme of conditions. It would be further advantageous to provide a mechanism for allowing those components of the mooring system which remain permanently on site to avoid damage from the conditions which prompted the tanker to move off location. It would be further advantageous to avoid the need to individually reconnect each mooring line to the tanker when the tanker returns to the mooring station. It would be yet further advantageous to provide a mooring system from which the vessel could be released on short notice to avoid the rapid increase in loading due to changing sea ice conditions.